68Ga-DOTATATE PET/CT Compared with 131I-MIBG SPECT/CT in the Evaluation of Neural Crest Tumors

Objective(s): 68Ga-DOTATATE positron emission tomography (PET)/computed tomography (CT) has shown promising results in imaging of neural crest tumors (NCT). Herein, we compared the performance of 68Ga-DOTATATE PET/CT and 131I-MIBG single photon emission computed tomography (SPECT)/CT in the initial diagnosis, staging and follow-up of patients with NCTs. Methods: Twenty-five patients (males:females=8:17; age range=2–71 years) with clinically proven or suspicious neuroblastoma, pheochromocytoma (PCC) or paraganglioma (PGL) were enrolled in this prospective study and underwent both 68Ga-DOTATATE PET/CT and 131I-MIBG SPECT/CT. A composite reference standard derived from histopathological information, together with anatomical and functional imaging findings, was used to validate the results. Imaging findings were assessed on a per-patient and on a per-lesion basis. Sensitivity and accuracy were assessed using McNemar’s test. Results: Referring to radiological imaging and histopathological findings as reference standard, 68Ga-DOTATATE and 131I-MIBG scans showed a sensitivity and accuracy of (100%, 96%) and (86.7%, 88%), respectively, on a per-patient basis. In PCC/PGL patients, on a per-patient basis, the sensitivity of 68Ga-DOTATATE was 100% and that of 131I-MIBG was 77.8%. In neuroblastoma patients, on a per-patient basis, the sensitivities of both 68Ga-DOTATATE and 131I-MIBG were 100%. Overall, in this patient cohort, 68Ga-DOTATATE PET/CT identified 52 lesions and 131I-MIBG SPECT/CT identified only 30 lesions. On a per-lesion analysis, 68Ga-DOTATATE was found to be superior to 131I-MIBG in detecting lesions in all anatomical locations, particularly osseous lesions. According to the McNemar test results, differences were not statistically significant. Conclusion: This relatively small patient cohort suggests 68Ga-DOTATATE PET/CT be superior to 131I-MIBG SPECT/CT in providing particularly valuable information for both primary staging and follow-up in patients with NCT.


Introduction
Neuroblastomas (NBs), ganglioneuroblastomas, pheochromocytomas (PCCs) and paragangliomas (PGLs) are acknowledged as neural crest tumors (NCTs), deriving from the sympathetic nervous system, which itself comprises collections of neuroepithelial cells scattered throughout the body, anywhere from the neck to the pelvis, containing numerous neurosecretory granules of catecholamines as their definitive common feature. The better differentiated tumors arising from this system are known as paragangliomas, containing numerous catecholamine granules. Paragangliomas of the adrenal medulla, believed to arise from chromaffin-producing cells, are regarded as pheochromocytomas. On the other hand, poorly differentiated tumors arising from precursors of this system are known as neuroblastomas (1). These tumors of neuroendocrine origin mainly produce catecholamines and related substances, resulting in their increased plasma and urinary levels. Metastases originating from the malignant forms of this group of tumors could be identified in soft tissue, bone, bone marrow, lymph nodes and in other organs, namely, liver and lung. The diagnosis and follow-up of NCTs is accomplished by measuring the level of serum/urine tumor markers (catecholamines, metanephrines) together with cross-sectional and functional imaging methods, providing anatomic information and predicting tumor localization and extent (2). Computed tomography (CT) and magnetic resonance imaging (MRI) are the morphologic imaging modalities of choice in localizing these tumors with high sensitivity in lesion detection, but lack of appropriate specificity due to the difficulties in distinguishing between tumors deriving from the sympathetic nervous system or other tumor entities (2,3). Although these imaging methods are often regarded as firstline imaging modalities, functional imaging exerts advantages of high sensitivity, very high specificity, and performance of whole-body scanning, and are thus commonly suggested to clarify equivocal lesions detected by anatomical imaging.
123/131 Iodine-metaiodobenzylguanidine ( 123/131 I-MIBG) is currently contemplated as the reference tracer for single photon emission computed tomography (SPECT). In addition, SPECT imaging for somatostatin receptor (SSTR) has been widely carried out in neuroendocrine tumors (NETs) and also in NB, using somatostatin analogue 111 Inpentetreotide (4). Considering the shortcomings of SPECT imaging, including its limited sensitivity and spatial resolution, and the long duration of image acquisition, positron emission tomography (PET) is becoming the leading functional modality in cancer imaging, on the basis of its better image resolution, shorter duration of the procedure, and more accurate quantification. A few clinical studies have described PET imaging in human NCTs, using metabolic tracers such as the glucose analogue 18 F-fluorodeoxyglucose (FDG) and the amino acid analogue 18 F-fluorodopa (5,6). PET imaging of SSTR could be performed using somatostatin analogues such as 68 Ga-[tetraxetan-D-Phe1, Tyr3]-octreotate ( 68 Ga-DOTATATE), a somatostatin analogue suited for PET imaging displaying very high SSTR2 receptor binding.
Herein we report a cohort of patients with NCTs and contemporaneous 131 I-MIBG SPECT/CT and 68 Ga-DOTATATE PET/CT imaging. The aim was to compare the performance of these functional imaging modalities in detecting malignant PCC, PGL and NB in the initial diagnosis and follow-up of these patients.

Study population
Twenty-five consecutive patients (males: females=8:17; age range 2-71 years; median age of 8 years) who underwent both 68 Ga-DOTATATE PET/CT and 131 I-MIBG SPECT/CT examinations either for primary diagnosis, initial staging, restaging, evaluation of metastases, evaluation of response to treatment, and surveillance of NCTs were recruited in this prospective study (Table 1). Among them, twenty-four patients had biochemically and histologically proven NCTs (fifteen neuroblastoma, eight PCCs, and one PGL) and one patient was diagnosed with von Hippel-Lindau disease (VHL) and was suspected to have PCC. Patients who met the following criteria were included in the investigation: (a) raised 24-h urine catecholamine, (b) persistent uncontrolled blood pressure while on more than two anti-hypertensive medications, (c) histologically proven metastatic disease for further restaging, (d) suspicion of metastasis according to the presence of suspicious metastatic lesion(s) on contrast-enhanced CT or MRI, and (e) primary adrenal mass of greater than 5 cm. All examinations took place within 30 days from each other with no therapeutic interventions between the two examinations. 10 Asia Ocean J Nucl Med Biol. 2020; 8(1):8-17. If necessary, young children were sedated to allow imaging. None of the patients was on medication known to interfere with the cellular uptake of catecholamines, comprising sympathomimetic amines, tricyclic antidepressants, or reserpine (7). It has been an interval of more than 4 weeks between the last injection of long-acting SST analogues and performance of 68 Ga-DOTATATE PET/CT scans. The study protocol was approved by the Ethics Committee of Tehran University of Medical Sciences and written informed consent was obtained from all patients or their guardians.

Radiopharmaceutical preparation
Ga-68 was obtained from a commercially 68 Ge/ 68 Ga (PARS-GalluGEN) generators, which is locally available in the country form Pars Isotope Company, Tehran, Iran. 131 I-MIBG and DOTA-Tyr 3 -Octreotate (DOTATATE) kit were also provided from this company. The radiopharmaceutical preparation ( 68 Ga-DOTATATE) and all quality controls were carried out according to the standard protocols provided by the kit manufacturer.

Ga-DOTATATE PET/CT imaging protocol
PET imaging was performed using Biograph 6 PET/CT scanner (Siemens Medical Solutions, Erlangen, Germany). Depending on the weight of the patients, 4-5 mCi of 68 Ga-DOTATATE was administered intravenously. Image acquisition was performed at 60-min post-injection with a whole-body field of view (vertex to mid-thigh), and patients were asked to empty their bladder. The CT exposure factors for all examinations were 80 mAs, 80-130 keV, pitch of 0.8 and slice thickness of 5 mm. maintaining patient position, a whole-body PET scan was performed immediately after CT acquisition and covered an area identical to that covered by CT. PET acquisition was carried out in 3D with 4 min per bed position. PET images were reconstructed using CT for attenuation correction. Using ordered subsets expectation maximization algorithm (four iterations and eight subsets), transaxial PET data were reconstructed.

I-MIBG SPECT/CT imaging protocol
To minimize 131 I uptake in the thyroid gland, 20 drops of Lugol's solution twice a day were administered orally 3 days prior to injection of 131 I-MIBG and continued 7 more days after the procedure. In young children, the administration of Lugol's solution was adapted to body weight. A dose of 37-45 MBq (1.0-1.2 mCi) of 131 IMIBG was injected intravenously. Standard whole-body planar images (simultaneous anterior and posterior views, 256×256 matrix, 5 cm/min) was carried out at 24 h and 72 h using a Siemens Symbia T1 gamma camera with a high-energy parallel-hole collimator with the patient supine. Using 120 projections (60 views x dual heads) at 25-35 s per view, SPECT of the cervicothoracic and abdominopelvic regions were performed over a 180° arc (clockwise rotation) and images were obtained as 128×128 matrix at a zoom factor of 1, using a step-and-shoot rotation with an autocontinuing non-circular orbit. Low dose CT was performed with the 72-h post injection acquisition for attenuation correction and anatomic orientation. SPECT images without (24 h after injection) and with attenuation correction (72 h after injection) were reconstructed iteratively.

Image interpretation
Two experienced nuclear medicine physicians who were aware of the patient's clinical history but blinded to any results of the anatomical imaging modalities, reviewed all 68 Ga-DOTATATE PET/CT and 131 I-MIBG SPECT/CT images on a Mac-based OsiriX workstation and an ESOFT Syngo workstation (Siemens Healthcare), respectively. The site and number of lesions were examined. A per-lesion and per-patient analysis were executed for both imaging modalities. If the results acquired by the two viewers were discordant, a third reader was consulted as a referee. Positive uptakes on PET findings were based on visual assessment and maximum standardized uptake values (SUVmax) with correction for body weight. Clear demarcation of the lesion with tracer accumulation higher than that of the liver and tracer uptake greater than physiological activity was implemented as the criteria for interpreting a scintigraphic lesion as malignant. 131 I-MIBG uptake in the adrenals was considered to be normal if it was mild, symmetrical and not enlarged. Both higher and distinct focus of increased tracer uptake from the background and from MIBG normal distribution or side asymmetry of the adrenal glands were considered pathological on MIBG images (8). All patients underwent further morphologic imaging using contrast-enhanced CT, MRI, or ultrasound during their clinical work-up. On a per-patient basis, scans were assumed to be positive provided that at least one malignant lesion (primary tumor and/or metastases) was seen, regardless of the number of foci, while, scans with no, mild or symmetrical uptake were considered to be negative. A composite reference standard was adopted to validate the findings by incorporating both histological proof of metastatic lesions together with anatomical and functional imaging information. At least one tumor site was confirmed histologically in all patients investigated. In patients with multiple tumor manifestation, not all sites were verified histologically because of the advanced stage of the 12 Asia Ocean J Nucl Med Biol. 2020; 8(1): 8-17. disease and systemic therapy as the first therapeutic choice. Morphologic imaging methods and the clinical and imaging follow-up were used so as to clarify discrepant results between 68 Ga-DOTATATE PET/CT and 131 I-MIBG SPECT/CT. Clinical and imaging follow-up was considered in case of clearly positive PET/CT or SPECT/CT but negative morphologic imaging. On the other hand, if either PET/CT or SPECT/CT was negative in a region with a clearly visible lesion in morphologic imaging, the negative finding was assumed to be false-negative. Analysis on a per-lesion basis was performed on the following areas: cervicothoracic soft tissue, abdominopelvic soft tissue, and bone. For sensitivity calculations, studies were considered either positive or negative.

Statistical analysis
Normal distribution was assessed by the Kolmogorov-Smirnov test. In the per-lesion analysis, histopathologically positive results or unequivocal focus/foci of abnormally increased tracer uptake detected by either 68 Ga-DOTATATE PET/CT or 131 I-MIBG SPECT/CT imaging in a nonphysiological site, confidently judged as disease by the reporting physicians were considered to be true positive. In cases in which the lesion detected by contrasted/non-contrasted CT or MRI showed no uptake on either 68 Ga-DOTATATE PET/CT or 131 I-MIBG SPECT/CT imaging was considered true-negative. The presence of at least one positive lesion per patient was assumed to be a positive finding for metastatic disease in the per-patient analysis. SPSS Software (version 24.0) was used to perform statistical analysis. Nonparametric methods were used. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and accuracy were calculated. The McNemar test was used to compare sensitivities, specificities, PPVs, NPVs and accuracies between the different diagnostic modalities. The number of lesions per patient was compared using the Wilcoxon signed-rank test.
One-way analysis of variance (ANOVA) was also carried out. A p value of less than 0.05 was considered statistically significant.

Per-lesion analysis
A total of 54 lesions of variable sizes ranging from 7.3 to 60.2 mm were detected when considering both 131 I-MIBG and 68 Ga-DOTATATE findings. 68 Ga-DOTATATE located 52 lesions, whereas 131 I-MIBG detected fewer lesions at 30 (Tables 1 and  2). Three of the lesions detected on 68 Ga-DOTATATE and one lesion observed on 131 I-MIBG were attributed to hepatic hydatid cyst and thus interpreted as false positive. The range of measured SUVmax over 68 Ga-DOTATATE uptake lesions was from 1.99 to 27.70 with a mean of 8.74±6.42. As we had no reference to check every single lesion as either true or false positive or negative, sensitivity and accuracy of these two modalities could not be assessed. As it is evident by Table 1 and 2, 68 Ga-DOTATATE detected more lesions than 131 I-MIBG across all anatomical locations ( Figure 1). This was particularly so with bony lesions where a total of 15 lesions were detected by 68 Ga-DOTATATE compared to just 5 by 131 I-MIBG ( Table 2). The number of lesions per patient in 68 Ga-DOTA-TATE PET/CT and 131 I-MIBG SPECT/CT suggested the superiority of the former modality in detecting metastatic lesions using the Wilcoxon signed-rank test (P value=0.004).

Discussion
In spite of the well-known limitations of MIBG scintigraphy, including the complicated patient preparation (including thyroid blockade and discontinuation of certain drugs), low spatial resolution, prolonged imaging protocol, physiological uptake restricting in detection of small lesions, and relatively high radiation dose to the patients, this imaging method has been regarded as a standard functional imaging for initial evaluation and management of a subgroup of NCTs, namely, PCC, PGL, and neuroblastoma (9). Notwithstanding the fact that 123 I-MIBG is considered as the standard in clinical practice due to the favorable characteristics of the labelling radioisotope 123 I, with its shorter half-life, lack of beta particulate emission, and higher emitted photon flux (10) as compared to 131 I, 123 I might not be available in every facility, and this results in the administration of 131 I, which suffers from low image quality and unfavorable dosimetry, instead. With the advent of the PET imaging technique and radiolabeling of somatostatin analogues with positron emitting isotopes, including 68 Ga, for imaging somatostatin receptor expressing tumors, 68 Ga-DOTATATE PET scan has demonstrated dramatic improvement of the spatial resolution and lesion detectability compared to MIBG scintigraphy (11). Additionally, 68 Ga-DOTATATE PET imaging exhibits both practical advantages (no patient preparation, easy synthesis, short and convenient (less than 2 hours) imaging procedure, lower radiation exposure (12) and wide availability due to 68 Ge/ 68 Ga generator), and added diagnostic information and quantification capability. This imaging modality has also been proven to have high impact on management of patients with neuroendocrine cancers, signifying its role in clinical practice of somatostatin-avid malignancies (13). In spite of the speculated superior diagnostic performance of PET with various radiopharmaceuticals in comparison with MIBG scintigraphy, based on cumulative official and clinical experiences, MIBG is still considered as the workhorse in evaluation of NCT. A potential drawback of 68 Ga-DOTATATE PET imaging could be the high cost of 68 Ge/ 68 Ga generators. However, recent approval of the SST analogue kit by the United States Food and Drug Administration along with the increasing demand for 68 Ga-labelled radiotracers will make 68 Ge/68Ga generators more readily available. In addition, imaging centralization and a referral system, as examples of an effective systematic planning, would help reduce the cost of 68 Ga imaging. Herein, we described 25 patients with neuroblastoma, PCC, and PGL with comparative 131 I-MIBG SPECT/CT and 68 Ga-DOTATATE PET/CT evaluations. This investigation aims to add to the recent small case series comparing 131 I-MIBG and 68 Ga-DOTATATE in the diagnosis and follow-up of NCTs. In line with previous studies (14,15), the observed diagnostic performance of 131 I-MIBG scintigraphy in the detection of metastatic PCC/PGL on per-patient and per-lesion basis was rather disappointing, although this was not the case with neuroblastoma. As 131 I-MIBG scintigraphy is found to remarkably underestimate the disease extent, its use as a diagnostic agent should be confined to evaluation of patient's eligibility for 131 I-MIBG therapy (16). Utility of 131 I-MIBG SPECT/CT has been advocated to increase the diagnostic certainty in indeterminate lesions, and better fair situation for comparison with 68 Ga-DOTATATE images, improving diagnostic performance compared to planar imaging (17). Findings of the current study demonstrated marginally better sensitivity and accuracy of 68 Ga-DOTATATE PET/CT compared to than 131 I-MIBG SPECT/CT on per-patient and perlesion bases, however, we could not detect any statistically significant superiority of 68 Ga-DOTATATE PET/CT over 131 I-MIBG SPECT/CT. Anatomical areas have been assumed to affect the detection rate in functional imaging studies (18). In our study, 68 Ga-DOTATATE identified an equal or greater number of patients with disease than 131 I-MIBG in all anatomical areas ( Table 2). The per-lesion comparison further revealed the impact of the anatomical distribution of disease on pick-up rate. 68 Ga-DOTATATE showed better sensitivity in all anatomical positions than 131 I-MIBG, except for the lesions in the abdominopelvic region. The superior diagnostic performance of 68 Ga-DOTATATE imaging was particularly noticeable for osseous lesions, in which 68 Ga-DOTATATE detected nearly threefold more lesions than 131 I-MIBG (15 vs. 5). This could be partly attributed to the smaller size of bony lesions, which are better distinguished by PET/CT due to its higher spatial resolution than SPECT/CT. Win et al. were the first to analyze and compare 68 Ga-DOTATATE and MIBG imaging in small group of patients with neuroectodermal tumors (19). They found 68 (22). Both Ga-DOTATATATE PET/CT and MIBG SPECT/CT correctly identified 9 primary tumors. Ga-DOTATATATE PET/CT was found to be able to detect associated extra-adrenal lesions not revealed by MIBG study in patients with multiple endocrine neoplasia syndrome. They suggested Ga-DOTATATA PET/CT as the nuclear medicine imaging of choice to evaluate suspected primary PCC/PGL, specifically in the situation of multiple endocrine neoplasia syndrome. Results of these previously performed investigations seem to suggest the diagnostic superiority of 68 Ga-DOTATATATE PET/CT over 123/131 I-MIBG scintigraphy in patients with NCTs; nevertheless, discordant findings using these radiopharmaceuticals are also reported.
It is evident that accurate mapping of all metastatic lesions could have potential implication in assignment of cancer staging, subsequent guidance in therapeutic approach, and finally as an important indicator in prognostication. Underestimation of tumor extent could potentially culminate in inappropriate management, particularly when choosing the appropriate mode of therapy between high risk metastasectomy, palliative chemotherapy or targeted radiotherapy. Moreover, comprehensive evaluation of all target lesions is required to assess therapeutic response, properly (23). The higher diagnostic performance of 68 Ga-DOTATATE PET/CT, on a per-lesion basis, in comparison with 131 I-MIBG scintigraphy may somehow have a potential impact on therapeutic decision making. Surprisingly, despite the poorer inherent characteristics of 131 I than 123 I for imaging with current gamma cameras, which contribute to poorer target-to-background ratio, our observed sensitivity of 131 I-MIBG scintigraphy in detecting metastatic NCTs is substantially higher than what was reported in other studies which used 123 I-MIBG scintigraphy (14,(24)(25)(26). Discrepancies observed in our results from those previously reported in other studies may be either related to the utilization of SPECT/CT for better characterization of the lesions, or the use of different reference standards in verifying the disease existence. In the current investigation, a composite reference standard derived from anatomical and functional imaging findings, together with histopathological information, has been adopted to overcome the imperfect standards of some of the previous studies, in which, CT or MRI, generally having low specificity and greater number of inconclusive results, particularly for small lesions, were served as the reference standard . This study has certain limitations that are required to be acknowledged. Firstly, our relatively small sample size, although justified by the rarity of the disease, hinders further analysis. Another limitation was our inability to perform genetic analysis so as to assess potential relationship between 68 Ga-DOTATATE PET/CT and 131 I-MIBG SPECT/CT results and genetic abnormalities, leading to a more comprehensive understanding of the correlation between germline mutation and tumor characteristics. As the justification of the diagnostic performance of an index test could be a strenuous task when the gold standard is imperfect, a composite reference standard has been adopted to produce more meaningful results in clinical practice; however, the possibility of reference-standard-related bias, comprising error and variation in image interpretation and misclassification of lesions, cannot be totally excluded. Notwithstanding the fact that disease extension in NCTs could be accurately examined by a multimodality approach using different radiotracers, this approach is time-consuming, impractical and not cost-effective. The marginally higher sensitivity and accuracy of 68 Ga-DOTATATE PET/CT compared to 131 I-MIBG, implies that somatostatin receptor PET/CT imaging may offer a better overview on tumor spread and extension and recommends 68 Ga-DOTATATE PET/CT into the standard algorithm of evaluation of such tumors. In cases with a high pretest probability of metastatic NCTs, 68 Ga-DOTATATE PET/CT should be considered as the preferred investigation to exclude active disease, when 131 I-MIBG scintigraphy does not detect lesions. In addition, 68 Ga-DOTATATE PET/CT should be considered in preference to 131 I-MIBG scan in NCTs when metastatic spread, particularly to the bone, is questioned. Moreover, as the high detectability of somatostatin receptor expression in metastatic NCTs provides the basis for consideration of peptide receptor radionuclide therapy (PRRT) as part of the multimodality management of this group of malignancies, 68 Ga-DOTATATE PET/CT, providing valuable information on somatostatin receptor status in this group of patients. Bearing all the abovementioned advantages of 68 Ga-DOTATATE PET/CT in mind, further large prospective and multicenter studies comparing this modality with MIBG SPECT/CT in this setting are required. 68 Ga-DOTATATE PET/CT could be superior to 131 I-MIBG SPECT/CT in providing particularly valuable information for both primary staging and follow-up in patients with neural crest tumors.